Abstract
ABSTRACT High levels of eutrophication in coastal lagoons due to human activity have been documented worldwide. Among the main impacts observed are anoxia, hypoxia, toxic algal blooms, fish kills, loss of biodiversity and loss of bathing. This study aimed to evaluate the evolution of the trophic state of Lagoa da Conceição, a subtropical lagoon located in an urbanized watershed on the island of Santa Catarina - Brazil. Spatio temporal patterns of stratification and eutrophication were investigated to understand the main biochemical changes over time. The water quality data were obtained from field campaigns supplemented with literature of the last 15 years. The vertical structure of the water column and the trophic state were evaluated by the stratification index and the TRIX index, respectively. Analyses of variance were performed in order to identify possible temporal variations in vertical stratification and trophic level. Eutrophication effects on biogeochemical cycles were verified through a multi-dimensional cluster analysis (MDS) and correlations between variables related to physical, chemical and biological processes were verified by principal component analysis (PCA). The results showed that the water column is homogeneous in all regions except in the central region of the lagoon, and the highest ammonia concentrations and lowest dissolved oxygen concentrations with periods of anoxia are observed in bottom waters. The study looked at the high trophic level of the lagoon and its inability to process the biogeochemical changes imposed by urban development.
Highlights
Destruction of marine ecosystems by anthropic action is a global phenomenon that is attaining alarming proportions (LOTZE et al, 2006; HALPERN et al, 2008, 2015), and one of the main causes of degradation in coastal zones is the input of nutrients from industry and agriculture (GESAMP, 2001)
Choked lagoons in particular are characterized by high residence time associated with low water renewal rates, which leads to the accumulation of nutrients, sediments and organic matter (KJERFVE, 1994; KNOPPERS; KJERFVE, 1999; MIRANDA; CASTRO; KJERFVE, 2002; EKAU; KNOPPERS, 2003; BIANCHI, 2006;)
The purpose of the present study was to systematize the existing water quality data for Lagoa da Conceiçao in order to: a) evaluate the lagoonal regions as to the existence or absence of stratification in the water column; b) evaluate the temporal evolution in the region where more intense stratification processes were identified; c) describe the spatial variations and identify tendencies toward eutrophication in the lagoon based on the Trophic State Index – trophic state index (TRIX) (VOLLENWEIDER et al, 1998) during the period from 2001 to 2015, and d) look at the dynamics of the inorganic nutrients, oxygen and phytoplankton biomass to understand the main biochemical modifications that occurred in the system during the period analyzed
Summary
Destruction of marine ecosystems by anthropic action is a global phenomenon that is attaining alarming proportions (LOTZE et al, 2006; HALPERN et al, 2008, 2015), and one of the main causes of degradation in coastal zones is the input of nutrients from industry and agriculture (GESAMP, 2001). Choked lagoons in particular are characterized by high residence time associated with low water renewal rates, which leads to the accumulation of nutrients, sediments and organic matter (KJERFVE, 1994; KNOPPERS; KJERFVE, 1999; MIRANDA; CASTRO; KJERFVE, 2002; EKAU; KNOPPERS, 2003; BIANCHI, 2006;) In these environments, the excessive discharge of nutrients causes changes that lead to increasing levels of eutrophication, resulting in a decline of biodiversity, changes in the food chains, harm to human health and economic and social losses that are difficult to measure (NIXON, 1995; GASALLA; ROSSI-WONGTSCHOWSKI, 2004; TUNDISI, 2003; RABALAIS et al, 2009). Population studies performed by Campanario (2007) indicate that the urban population around the lagoon grew 93.2% between 2001 and 2015, at a rate of 6.3% a year, a growth rate that is four times higher than the 1.5% a year of the state of Santa Catarina during the same period (CAMPANARIO, 2007)
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